The need for environmentally sustainable housing and cities is a major factor driving wind energy conversion systems for the built environment. One consequence of this is the reemergence of cross-wind-axis machines, most often oriented vertically and termed vertical axis wind turbines (VAWTs).
Much research was conducted on these machines until the 1980's, but with the increasing success of horizontal axis wind turbines (HAWTs), it was largely discontinued. Nevertheless, for the built environment, VAWTs have several advantages over HAWTs, namely: low sound emission (due to lower tip speed ratios), better esthetics do the VAWTs' three dimensionality, insensitivity to yaw, and increased performance in skew (see Ferreira S M., van Bussel G., Scarano F., Kuik G., “2D PIV Visulization of Dynamic Stall on a Vertical Axis Wind Turbine”, AIAA Paper 2007-1366. 45th AIAA Aerospace Sciences Meeting and Exhibit, 8-11 Jan. 2007, Reno, Nev.).
International Patent Publication WO 2009/053984 discloses the earlier technique of the same inventor relating to a fan or horizontal axis wind turbine comprising at least one blade, and at least one plasma actuator mounted on said blade. This technique provides performance improvements or energy savings for fans used in such applications as personal, industrial and automotive cooling, ventilation, vacuuming and dust removal, inflating, computer component cooling, propulsors for unmanned and manned air vehicles, propulsors for airboats, air-cushion vehicles, airships and model aircraft. Additionally, the invention provides higher performance such as higher lift and higher lift efficiency to small air vehicles. These advantages are achieved by using plasma actuators to provide active flow control effectors into thin blades and wing.
U.S. Pat. No. 7,537,182, of the same inventor, discloses a method of controlling a shear layer for a fluid dynamic body. According to this method, first periodic disturbances are introduced into the fluid medium at a first flow separation location, and simultaneously second periodic disturbances are introduced into the fluid medium at a second flow separation location. A phase difference between the first and second periodic disturbances is adjusted to control the flow separation of the shear layer as the fluid medium moves over the fluid dynamic body.
U.S. Pat. No. 6,267,331, of the same inventor, discloses a method for inhibiting dynamic stall of an airfoil by causing a fluid to flow out of at least one location on the airfoil. This location may be anywhere on the airfoil; but if the location is within one-quarter of the airfoil chord from the leading edge and the fluid flow has non-zero net mass flux, then the fluid flow is modulated at a frequency described by a Strouhal ratio greater than one.
U.S. Pat. No. 4,504,192 discloses an air jet spoiler arrangement for a Darrieus-type vertical axis wind-powered turbine. Air is drawn into hollow turbine blades through air inlets at the ends thereof and is ejected in the form of air jets through small holes or openings provided along the lengths of the blades. The air jets create flow separation at the surfaces of the turbine blades, thereby inducing stall conditions and reducing the output power. A feedback control unit senses the power output of the turbine and controls the amount of air drawn into the air inlets accordingly.
Some problems associated with the flow separation and solutions for reducing the same are described in the following publications of the same inventor: “The control of flow separation by periodic excitation”, Greenblatt, D. and Wygnanski, I., Progress in Aerospace Sciences, Volume 36, Number 7, October 2000, pp. 487-545(59); “Effect of leading-edge curvature on separation control: A comparison of two NACA airfoils”, Greenblatt, D. and Wygnanski, I., 40th AIAA Aerospace Sciences Meeting and Exhibit Reno, Nev.”, January 2002; “Effect of leading-edge curvature on airfoil separation control”, Greenblatt, D. and Wygnanski, I., AIAA Journal of Aircraft, Vol. 40, No. 3, 2003, pp. 473-481.